The present invention relates to a method for preparing 1,4-naphthoquinone (hereinafter merely referred to as naphthoquinone) which is useful as a starting material for the synthesis of various dyes and organic compounds. More particularly it relates to a method for the preparation of naphthoquinone by catalytic vapor phase oxidation of naphthalene as well as a catalyst suitable for use in such a method and processes for the preparation of such a catalyst.
Naphthoquinone is generally prepared by vapor phase oxidation of naphthalene with a molecular oxygen-containing gas in the presence of a catalyst in a fixed bed-type reactor to form 1,4-naphthoquinone. A catalyst which has been proposed as effective for this purpose is prepared by mixing active ingredients comprising vanadium pentoxide, potassium sulfate and potassium pyrosulfate with a carrier material such as silicic acid or diatomaceous earth and then shaping and calcining the mixture.
For example, Japanese Patent Publication No. 5533/1976 discloses the use of a ternary system catalyst for the preparation of naphthoquinone comprising potassium sulfate, potassium pyrosulfate and vanadium pentoxide on a carrier wherein the pore size distribution of the carrier is controlled so as to improve the catalytic activity. Japanese Patent Publication No. 15063/1968 discloses the use of a similar catalyst on a carrier which has been calcined at a high temperature prior to supporting the active ingredients thereon. It also discloses that the catalyst may optionally contain tungsten to further improve the activity. Other catalysts of the above basic ternary system known in the art include one whose carrier has a specific pore volume and pore size distribution (Japanese Patent Laid-Open Application No. 34353/1972), one comprising a carrier in agglomerated form prepared by washing diatomaceous earth with an acid (Japanese Patent Publication No. 22559/1976), and one further containing iron as an essential element to improve the yield of the naphthoquinone product.
These prior art catalysts for the preparation of naphthoquinone have many problems. The conversion of a catalyst of this type is not compatible with the selectivity thereof toward the desired naphthoquinone; a catalyst capable of providing a higher conversion of naphthalene in order to produce naphthoquinone with a higher yield tends to have decreased selectivity, while a catalyst capable of producing naphthoquinone with a higher selectivity tends to have decreased conversion so that the amount of unreacted naphthalene is increased, which is accompanied by an increased load in a succeeding separation step for isolating the reaction product from the unreacted naphthalene. Moreover, with these catalysts, if one can obtain increased conversion of naphthalene to improve the yield of naphthoquinone, the reaction must be conducted at a relatively high temperature in the range of from 420.degree.to 450.degree. C., which inevitably makes the temperature control complicated.
As the reaction temperature increases, undesirable consecutive oxidative reactions of the naphthoquinone product may occur more readily, resulting in the formation of by-products such as phthalic anhydride, maleic anhydride, CO, and CO.sub.2 in increased amounts, thereby decreasing the yield of naphthoquinone. It is said that the presence of sulfur in the catalyst in the form of S.sub.2 O.sub.7 or similar solid sulfur oxide is essential for the formation of naphthoquinone from naphthalene. However, a loss of sulfur by escaping out of the system as gaseous SO.sub.x increases with increasing reaction temperature, thereby varying the composition of the catalyst and decreasing the selectivity toward naphthoquinone. Therefore, if a relatively high reaction temperature is employed, it may be necessary to carefully control the temperature such that the temperature does not rise significantly beyond the predetermined range, which involves rather complicated operation.